Materials used for high temperature applications must have a good stability in an oxidizing atmosphere, and good mechanical properties. In addition, materials used for electrodes in electrochemical processes in molten electrolytes must further have good electrical conductivity and be able to operate for prolonged periods of time under polarizing conditions. At the same time, materials used on an industrial scale should be such that their welding and machining do not present unsurmountable problems to the practitioner. It is well known that ceramic materials have good chemical corrosion properties. However, their low electrical conductivity and difficulties of making mechanical and electrical contact as well as difficulties in shaping and machining these materials seriously limit their use.
In an attempt to resolve well known difficulties with conductivity and machining of ceramic materials, the use of cermets was proposed. Cermets may be obtained by pressing and sintering mixtures of ceramic powders with metal powders. Cermets with good stability, good electrical conductivity and good mechanical properties, however, are difficult to make and their production on an industrial scale is problematic. Also, the chemical incompatibilities of ceramics with metals at high temperatures still present problems. Composite materials consisting of a metallic core inserted into a premachined ceramic structure, or a metallic structure coated with a ceramic layer have also been proposed.
U.S. Pat. No. 4,374,050 discloses inert electrodes for aluminum production fabricated from at least two metals or metal compounds to provide a combination metal compound. For example, an alloy of two or more metals can be surface oxidized to form a compounded oxide of the metals at the surface on an unoxidized alloy substrate. U.S. Pat. 4,374,761 discloses similar compositions further comprising a dispersed metal powder in an attempt to improve conductivity. U.S. Pat. Nos. 4,399,008 and 4,478,693 provide various combinations of metal oxide compositions which may be applied as a preformed oxide composition on a metal substrate by cladding or plasma spraying. The direct application of oxides by these application techniques, however, is known to involve difficulties. Finally, U.S. Pat. No. 4,620,905 describes an oxidized alloy electrode based on tin or copper with nickel, iron, silver, zinc, mangnesium, aluminum or yttrium, either as a cermet or partially oxidized at its surface. Such partially oxidized alloys suffer serious disadvatages in that the oxide layers formed are far too porous to oxygen, and not sufficently stable in corrosive environments. In addition, it has been observed that at high temperatures the partially oxidized structures continue to oxidize, and this uncontrolled oxidation causes subsequent segregation of the metal and/or oxide layer. In addition, the machining of ceramics and achieving a good mechanical and electrical contact with such materials involves problems which are difficult to solve. Adherence at the ceramic-metal interfaces is particularly difficult to achieve and this very problem has hampered use of such simple composites.